Braking apparatus and method of controlling the same

ABSTRACT

A braking apparatus comprises a first contact unit arranged on the rotary shaft side of a wheel, at least a second contact unit for contacting the first contact unit for generating a friction force, an actuator for pressing the second contact unit against the first contact unit, and a control unit for controlling the operation of the actuator, wherein the control unit performs the position control operation for moving the second contact unit in such a manner that the interval between the first contact unit and the second contact unit is smaller than a preset distance, and the force control operation for pressing the second contact unit against the first contact unit in accordance with the required braking force.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part (CIP) of U.S.application Ser. No. 09/531,463, filed Mar. 20, 2000, and makesreference to, incorporates the same herein, and claims all benefitsunder 35 U.S.C. §120 from the U.S. application Ser. No. 09/531,463.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a braking apparatus for brakinga rotary unit by bringing a press unit on an actuator into contact withthe rotary unit and a method of controlling the braking apparatus.

[0003] A brake fluid pressure control system intended for anti-skidcontrol or traction control is disclosed in JP-A-5-147524, in which thebrake cylinder pressure is directly changed by operating a pump and thefluid pressure of the brake cylinder is controlled to a level notdirectly related to the operating condition of the brake pedal. In sucha system, a gear pump is interposed between a brake master cylinder anda brake cylinder, the pump is normally in idle runing not to prevent thesupply of the fluid pressure of the master cylinder, so that in the casewhere the pressure reduction is required as at the time of anti-skidcontrol, the motor of the gear pump is driven to return the brake fluidto the master cylinder, while at the time of pressure increase, themotor is driven reversely to supply the brake fluid to the brakecylinder.

[0004] Another braking system comprising a brake pedal and a brakecylinder not directly connected to each other is disclosed inJP-A-10-203338, in which the brake cylinder pressure is changed directlyby the operation of a pump. This system comprises a bypass circuit inparallel to the pump for supplying the brake fluid to the brake cylinderfrom a tank, and the brake fluid pressure is easily controlled byadjusting the opening of a variable orifice arranged midway of thecircuit. Specifically, when adjusting the fluid pressure in the brakecylinder, the opening of the variable orifice but not the pumprevolution speed or torque is increased thereby to reduce the fluidpressure for a smaller opening degree, thus controlling the fluidpressure upward.

[0005] In any one of the conventional braking systems described above,the pump or the variable orifice is controlled so that the brake fluidpressure coincides with a pressure corresponding to the required brakingforce.

[0006] In the normal braking apparatus utilizing the friction, however,a gap (pad clearance) is provided between a rotary member such as abrake disk and friction members such as brake pads to prevent thedragging of the friction member when the braking force is not required.As long as the brake pads are moving over the distance corresponding tothe pad clearance, however, the braking force is not obtained and thevehicle runs free. Therefore, it is necessary to bring the brake padsinto contact with the brake disk as early as possible. Also, the brakingforce is determined by the pressure exerted by the brake pads, i.e. thebraking fluid pressure in the brake cylinder in the case of the fluidpressure type. For controlling the braking force, therefore, the fluidpressure is required to be controlled accurately.

[0007] According to the brake control system disclosed in JP-A-5-147524,the oil pressure supplied to the brake cylinder is generated directly bythe pump, for example, the piston can move freely while the brake padsare moving over the distance corresponding to the pad clearance. Thus,the piston continues to move while generating the pressure due to thesliding resistance and the fluid resistance with the brake cylinderuntil the brake fluid in an amount corresponding to the pad clearance iscompletely supplied. From the very instant the brake pads come intocontact with the brake disk, however, the piston position is restricted,and the flow rate of the brake fluid, which is considered anon-compressive liquid, into the brake cylinder is reduced to zero. As aresult, the brake fluid pressure increases sharply due to the force ofinertia of the pump, the motor and the brake fluid and the delayed fluidpressure control. The degree of this sharp increase in fluid pressuredepends on the flow rate, i.e. the motor revolution speed at the momentwhen the brake pads come into contact with the brake disk, leading tothe problem that the higher the motor speed, the shorter the time whenthe vehicle runs free while the fluid pressure increases more sharply.

[0008] In the case where the pressure is controlled by a variableorifice as shown in the other prior art JP-A-10-203338, in contrast, thebrake fluid pressure is not increased sharply but the piston travelspeed changes with a pressure command value. When a small braking forceis required, therefore, the pressure command value is small and the timeof free run lengthens correspondingly.

SUMMARY OF THE INVENTION

[0009] The object of the present invention is to provide a brakingapparatus high in responsiveness with the braking force controllablewith high accuracy, and a method of controlling the braking apparatus.

[0010] In order to achieve the object described above, according to thepresent invention, there is provided a braking apparatus comprising afirst contact unit arranged on a rotary shaft of a wheel, a secondcontact unit brought into contact with the first contact unit forgenerating the friction force, an actuator for pressing the secondcontact unit against the first contact unit, and a control unit forcontrolling the operation of the actuator, wherein the control unitperforms the position control operation for moving the second contactunit to such an extent that the interval between the first and secondcontact units is smaller than a preset distance (clearance) and thebraking force control operation for controlling the pressure exerted bythe second contact unit against the first contact unit.

[0011] In the position control operation, the second friction unit isdesirably set closer to the first friction unit by an amountcorresponding to the preset clearance.

[0012] According to this method, the second control unit is set closerto the first contact unit by the position control operation, and theforce of pressing the second contact unit against the first contact unitis controlled by the braking force control operation. Thus, the delaytime of generation of the braking force is shortened and the accuracy ofthe braking force control operation is improved at the same time.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a diagram showing a configuration according to a firstembodiment of the invention.

[0014]FIG. 2 is a diagram showing the processing flow for initializationof a braking apparatus according to the invention.

[0015]FIG. 3 is a diagram showing the processing flow for the brakingoperation of a braking apparatus according to the invention.

[0016]FIG. 4 is a diagram showing a configuration of a second embodimentusing a fluid pressure system as driving means for the braking apparatusaccording to the invention.

[0017]FIG. 5 is a schematic diagram showing a chronological change ofthe pump flow rate and the brake cylinder fluid pressure in the casewhere only the fluid pressure is controlled in response to a brakingcommand.

[0018]FIG. 6 is a schematic diagram showing a chronological change ofthe pump flow rate and the brake cylinder fluid pressure in the casewhere the braking control operation is performed according to theinvention.

[0019]FIG. 7 is a diagram showing a configuration of a third embodimentusing a pad position sensor as pad position detection means according tothe invention.

[0020]FIG. 8 shows a schematic diagram of a configuration according toanother embodiment of the invention.

DESCRIPTION OF THE EMBODIMENTS

[0021]FIG. 1 is a diagram showing a configuration according to a firstembodiment of the invention. In FIG. 1, a disk brake is schematicallyshown. Nevertheless, the invention is applicable with equal effect alsoto other brakes of friction type including a drum brake.

[0022] In FIG. 1, numeral 1 designates an actuator for driving contactmembers (hereinafter referred to as the brake pads) 2 pressed against acontact unit (hereinafter referred to as the brake disk) 3 fixed on theshaft of the wheel. The actuator 1 includes, for example, a linear ballactuator driven by a motor for converting the turning effort of themotor into a reciprocal motion through a ball screw mechanism.

[0023] In the aforementioned case, the operation of bringing the brakepads 2 to the vicinity of and pressing them against the brake disk 3 andthe operation of returning the brake pads 2 can be performed by forwardand reverse rotation of the motor. The braking force is determined bythe pressure exerted by the brake pads 2. The braking force, therefore,can be measured by a force sensor or can be determined from the torqueof the motor constituting the actuator 1, i.e. the magnitude of thecurrent flowing in the motor.

[0024] The position of the brake pads 2, on the other hand, can bedetermined by counting the number of revolutions of the motor fordriving the actuator 1.

[0025] According to this embodiment, therefore, a counter 87 fordetecting the revolution speed of the motor and a current detector 88for detecting the current flowing in the motor are used as a means fordetecting the relative position of the pads.

[0026] The two types of information described above are input to acontrol unit 10. When the brake pedal 4 is depressed by the driver, forexample, the required braking force is determined by the force ofdepressing the brake pedal 4. The control unit 10 controls the motor ofthe actuator 1 so that the pad pressure corresponds to the requiredbraking force. For this purpose, the brake pedal 4 includes a forcedetector 4 a for detecting the depression force and converting it to anelectrical signal.

[0027] According to this embodiment, the brake pads 2 are set to aposition with a preset pad clearance when the braking apparatus isinitialized. An example of the initialized state of the brakingapparatus is the time when the vehicle engine is started or the parkingbrake is released. To detect this state, an ignition switch 5 or aparking brake switch 6 is connected to the control unit 10.

[0028] Upon determination from these conditions that the brakingapparatus is initialized, a pad clearance setting unit 101 sets the padclearance according to the steps shown in FIG. 2. First, the position atwhich the brake pads 2 come into contact with the brake disk 3 isdetermined. In determining this position, as shown in FIG. 2, the brakepads 2 are slowly advanced until the position thereof comes to remainunchanged. This particular position of the brake pads 2 is the one wherethe brake pads 2 comes into contact.

[0029] Once the brake pads 2 come into contact with the brake disk 3,the required torque increases, and therefore the motor drive currentvalue also increases. Thus, the contact between the brake pads 2 and thebrake disk 3 can be detected by detecting the change in the current by acurrent detector 88.

[0030] Assume that the pad clearance is set as designated at step 22 inFIG. 2. Specifically, the pad clearance can be set by rotating the motorin reverse direction by an amount corresponding to the pad clearance seby the pad clearance memory 102 in FIG. 1. The pad clearance memory 102can be any memory device used for storing the pad clearance.

[0031] In setting the pad clearance, a mechanic may adjust the positionwhere the brake pads 2 come into contact with the brake disk 3, and fromthis position, set the brake pads 2 backward by an amount correspondingto a preset clearance. In this way, a superior relation between theorigin and the clearance can be maintained. An adjusting mechanism forthis purpose can be included in the actuator. In such a case, however,any change which may occur before the actual maintenance time cannot becorrected.

[0032] When the driver starts operating the braking apparatus bydepressing the brake pedal 4, for example, the control operation isperformed according to the procedure shown in FIG. 3.

[0033] At the time of braking operation, the brake pads 2 are broughtinto contact with the brake disk 3 as designated at step 30 in FIG. 3,and then the motor torque is controlled as designated at steps 31 to 32in FIG. 3.

[0034] This control operation is started with the position control inwhich the brake pads 2 are set closer to the brake disk 3, followed bythe force control in which the brake pads 2 are pressed against thebrake disk 3. Thus, the delay time of the braking force generation canbe shortened and the accuracy of controlling the braking force can beimproved.

[0035] Further, when terminating the braking force control upondiscontinuation of the braking force demand input, as at step 33 in FIG.3, the pad clearance is set again as in the initialization.Specifically, a command is issued from the pad position control unit 103to reverse the motor operation by an amount corresponding to the padclearance set by the pad clearance memory 102 in FIG. 1, thereby toseparate the brake pads 2 from the brake disk 3. The revolution speedfor this driving operation is equal to that of the reverse rotation forinitialization, and therefore without a sensor or the like for measuringthe position of the brake pads, the position at which the brake pads 2contact the brake disk 3 can be determined.

[0036] Once the contact position is determined, even in the case wherethe brake pads 2 are driven at maximum speed for shortening the free rundistance, the brake pads 2 can be brought into soft contact by reducingthe motor driving power immediately before coming into contact. Thus, itis possible to avoid the sharp increase of the contacting force of thebrake pads, i.e. the sharp increase of the braking force due to theinertia or the delayed control of the actuator 1.

[0037] After that, the command is switched to the braking pressurecontrol unit 104, and the brake pad pressure is controlled until thedisappearance of the braking force demand input. Further, whenterminating the brake pad pressure control upon disappearance of thebraking force demand input, the pad clearance is set again as at thetime of initialization, thereby making it possible to keep a constantclearance of the brake pads 2.

[0038] The first embodiment using a linear ball actuator has beendescribed above. Actually, however, the invention is applicable to alltypes of friction braking apparatus as well as the braking apparatuswith pad drive means.

[0039] Now, a second embodiment of the invention will be explained.

[0040]FIG. 4 is a diagram showing a configuration of a braking apparatusof fluid pressure type. In FIG. 4, reference numeral 41 designates areservoir tank for storing the brake fluid. The reservoir tank 41 isconnected to a brake cylinder 43 through a reversible pump 42. The pump42 is driven by a motor 44. The motor 44 driven in forward or reversedirection permits the pressuring operation in which the brake fluid issupplied from the reservoir tank to the brake cylinder 43 and thepressure reducing operation in which the brake fluid is returned fromthe brake cylinder 43 to the reservoir tank 41.

[0041] In the pressuring operation, the piston 45 is pushed out underthe pressure of the brake fluid supplied by the pump 42, and the brakingforce is generated as the brake pads 2 are pressed against the brakedisk 3. The braking force thus generated is determined by the brakefluid pressure, and is controlled by controlling the brake fluidpressure. The brake fluid pressure can be determined also from thetorque of the motor 44 for driving the pump 42, i.e. the magnitude ofthe current supplied to the motor 44. As shown in FIG. 4, a pressuresensor 46 can be used alternatively. By the way, the distance covered bythe brake pads 2 can be determined from the brake fluid flow ratedetector 47.

[0042] The aforementioned two types of information are input to thecontrol unit 10. When the driver depresses the brake pedal 4 under thiscondition, the required braking force is determined by the force ofdepression of the brake pedal 4. The control unit 10 controls the motor44 to secure a brake fluid pressure corresponding to this requiredbraking force.

[0043] According to this embodiment, the brake pad position is set witha predetermined pad clearance like in the first embodiment describedabove at the time of initializing the braking apparatus in the sameprocedure as in FIG. 2. As a method of determining the position wherethe brake pads 2 come into contact with the brake disk 3, the pump 42 isdriven slowly to advance the brake pads 2 slowly, and the position ofthe brake pads 2 at the instant when the internal pressure of the brakecylinder 43 rises is determined as the position where the brake pads 2and the brake disk 3 come into contact with each other. Thus, theposition where the brake pads 2 and the brake disk 3 come into contactwith each other can be detected either by the pressure sensor 46 or bydetecting the drive current value of the motor 44.

[0044] In setting the pad clearance, the pump 42 is driven in reversedirection by the fluid amount determined by the pad clearance based onthe information of the flow rate detector 47 in FIG. 4, and the brakefluid in the brake cylinder 43 is returned to the reservoir tank 41.

[0045] The next starting operation of the braking apparatus iscontrolled in the same manner as in FIG. 3 described above.Specifically, first, the pump 42 is driven in response to a command fromthe pad position control unit 103 in such a manner as to supply thebrake cylinder 43 with a fluid amount corresponding to the pad clearanceset by the memory 102 in FIG. 1 based on the information of the flowrate detector 47 in FIG. 4, and thus the brake pads 2 are brought intocontact with the brake disk 3. The fluid amount corresponding to the padclearance is equal tot he brake fluid amount returned to the reservoirtank 41 at the time of initialization. For this reason, the positionwhere the brake pads 2 come into contact with the brake disk 3 can bedetermined without any sensor for measuring the position of the brakepads 2. Once the position of contact is known, the brake pads 2 can bebrought into contact softly by weakening the driving force of the pump42 immediately before contact in the same manner as in the firstembodiment. Thus, the sharp increase in the brake fluid pressure can beavoided which otherwise might be caused by the inertia or the delayedcontrol of the motor 44, the pump 42 and the brake fluid.

[0046]FIG. 5 shows the relation between the brake cylinder fluidpressure P(a) and the pump flow rate Q(b) with time in the conventionalbraking apparatus, and indicates the increased free-run time due to theshortage of the flow rate before contact between the brake pads 2 andthe brake disk 3, and also the sharp increase in fluid pressure due tothe delayed fluid pressure control.

[0047]FIG. 6 is a schematic diagram showing the relation between thebrake fluid pressure P(a) and the pump flow rate Q(b) with time in thebrake control operation according to the present embodiment. As comparedwith FIG. 5, the free-run time is shortened while at the same timestabilizing the fluid pressure. Subsequently, the command is switched tothe braking pressure control unit 104 and the brake fluid pressurecontinues to be controlled until the disappearance of the input brakingforce demand. Further, at the end of the brake fluid pressure controlupon the disappearance of the braking force demand, the pad clearance isset again as at the time of initialization, so that an always constantclearance of the brake pads 2 is maintained.

[0048] In the fluid pressure system described above, the amount of thebrake fluid moved by the pump 42 can be determined by counting thenumber of revolutions of the pump shaft or the motor shaft when drivingthe pump. Specifically, since the displacement per pump revolution isknown, the brake fluid amount can be calculated as the product of thepump revolution speed and the displacement. Thus, the flow rate detector47 shown in FIG. 4 is configured with a revolution speed detector formeasuring the revolution speed of the motor 44. The revolution speedcorresponding to the clearance can specified by dividing the volumecorresponding to the pad clearance by the pump displacement. Thus, usingthe revolution speed as a command value from the memory 102, the outputof the revolution speed detector (flow rate detector) 47 can becontrolled by comparing it with the command value from the memory 102.For detecting the revolution speed of the motor 44, an encoder ismounted on the shaft to count the revolution speed directly, or therevolution speed can be counted from the number of pulsations of thedriving current value.

[0049] Now, a third embodiment of the invention will be explained.

[0050]FIG. 7 is a diagram showing a configuration comprising a counter87′ serving as a pad position sensor for measuring the position of thebrake pads 2, which is capable of measuring, either directly orindirectly, the position where the brake pads 2 come into contact withthe brake disk 3. With the counter (pad position sensor) 87′, theposition where the brake pads 2 come into contact with the brake disk 3is stored as an origin position memory 102A, and the pad position with aspecified pad clearance is stored in the clearance position memory 102B.At the time of initialization or at the end of the braking operation,the pad clearance setting unit 101 compares the output of the counter(pad position sensor) 87′ with the clearance position stored in thememory 102B, while when the brake pads 2 are brought into contact withthe brake disk 3 at the time of starting the braking operation, the padposition control unit 103 compares the output of the counter (padposition sensor) 87′ with the original position stored in the memory102A for the purpose of control.

[0051] Also, this embodiment, unlike the second embodiment relating tothe brake of fluid pressure type in which each wheel has an independentpump, may be applicable to a braking apparatus in which a fluid pressuresource is configured with a single pump and the fluid pressure issupplied to a plurality of wheels by valve control. In such a case, thepad clearance for each wheel is secured by a clearance mechanism builtin each pad driving unit like in the normal braking apparatus. Thus, theclearance setting unit 101 measures the pad clearance of each wheelwithout controlling the clearance amount positively, and stores theclearance position in the memory 102B. When the brake pads 2 are broughtinto contact with the brake disk 3 at the time of starting the brakingoperation, the pad position control unit 103 compares the output of thecounter (pad position sensor) 87′ with the origin in the memory 102A, sothat the brake pads 2 are advanced by controlling the valve forsupplying the brake fluid pressure for each wheel.

[0052] The embodiments described above concern an example of a series ofoperation performed upon receipt of a braking force demand. In the casewhere the braking operation by the driver is predicted upon detection ofthe following distance or an obstacle position by means of a radar orthe like, the clearance is closed by operating the pad position controlunit 103 in advance in order to quickly respond to the braking demand ofthe driver, and when the braking demand is received, the pressuringoperation is started immediately by the braking pressure control unit104.

[0053]FIG. 8 shows a schematic diagram of the control unit 10 accordingto another embodiment of the present invention. As shown in FIG. 8, thecontrol unit 10 can be applied to any of the brake systems shown inFIGS. 1, 4 and 7. This control unit 10 provides mechanics in maintenanceshops with easy maintenance and repair work. During maintenance andrepair work for conventional braking systems of vehicles, such asreplacing brake pad and overhaul, a mechanic moves back a piston ofcaliper assembly with his or her hand to separate a brake pads from abrake disk. However, since an actuator 1 or pump 42 is directlyinstalled on the caliper assembly in the brake system of the presentinvention, it is very difficult to move the brake pads 2 by hand.According to the embodiment shown in FIG. 8, the control unit 10automatically controls the actuator 1 or pump 42 to move a brake pads tobe separated from a brake disk at a longer distance (wider padclearance) to provide sufficient room for maintenance and repair work.

[0054] The control unit 10 includes a mode select switch 105 whichselects either one of a running mode and a maintenance mode, and amemory unit 102 which has at least two memories 102-1 and 102-2 to storetwo different pad clearance values. The memory 102-1 stores a padclearance value for the running mode wherein braking operation isnormally made in running or stopping of the vehicle. The memory 102-2stores a wider pad clearance value for the maintenance mode. Furthermemories can be included in the pad clearance memory unit 102 to storeother different pad clearance values utilized for other purposes. Thememory unit 102 may be a single memory device and the memories 102-1 and102-2 are different memory areas arranged in the single memory device.Alternatively, the memories 102-1 and 102-2 may be separate memorydevices.

[0055] In order to replace worn brake pads or overhaul the brake system,the ignition switch 5 is turned off and the maintenance mode is selectedby the mode select switch 105. As a measure of safety, the maintenancemode position of the select switch 105 can not be selected unless theignition switch 5 is turned off. When the maintenance mode is selected,the wide pad clearance value is read out from the memory 102-2 andsupplied to the clearance setting unit 101. In response to the wide padclearance value, the clearance setting unit 101 outputs a command valueto the actuator 1 to move back the brake pads 2 to be separated from abrake disk at the wider pad clearance to provide sufficient room formaintenance and repair work. In the case that the control unit 10 shownin FIG. 8 is applied to the brake system shown in FIG. 1 or FIG. 7, thecommand value of the maintenance mode from the clearance setting unit101 represents a positional value of the actuator 1 corresponding to thewide pad clearance value in the memory 102-2. In the case that thecontrol unit 10 shown in FIG. 8 is applied to the brake system shown inFIG. 4, the command value of the maintenance mode from the clearancesetting unit 101 represents an oil amount value or the number ofrevolutions of the pump 42 corresponding to the wide pad clearance valuein the memory 102-2.

[0056] Alternatively, when the control unit 10 shown in FIG. 8 isapplied to the brake system shown in FIG. 7, the command value of themaintenance mode from the clearance setting unit 101 may represent thenumber of revolution of the actuator motor 1 corresponding to the widepad clearance value in the memory 102-2. In such case, a revolutioncounter (not shown) is used for counting the motor revolution.

[0057] When the running mode is selected by the mode select switch 105,the control procedure of the brake pad clearance is same as that of theother example embodiments shown in FIGS. 1, 4 and 7.

[0058] The control unit 10 included in each of the embodiments describedabove can be realized with a well-known microcomputer system. Themicrocomputer system (not shown) includes a memory unit for storing adata table or a brake control program described with reference to theflowchart of FIGS. 2, 3, an input/output port for inputting/outputting asignal, a RAM for temporarily storing the processed data, a CPU and abus line connecting them. The memory unit for storing the controlprogram is a semiconductor memory, for example. An optical disk unit ora magnetic memory unit can also be used in place of the semiconductormemory as far as they can record computer readable program codes.

[0059] The features of the respective embodiments can be summarized asfollows.

[0060] (1) The braking apparatus comprises a pad clearance setting unitfor maintaining a preset clearance of the brake pads at the time ofinitialization and at the end of the braking operation, a pad positioncontrol unit for moving the brake pads of the braking apparatus to aposition in contact with the brake rotor when starting the brakingoperation, and a braking force control unit for controlling the force ofpressing the brake pads, wherein the brake pad position is controlled sothat a specified pad clearance is secured in response to a command fromthe pad clearing setting unit at the time of brake initialization. Whenperforming the braking operation, on the other hand, the brake pads areadvanced by an amount corresponding to the pads clearance in response toa command from the pad position control unit into contact with the brakerotor, followed by switching the command to the braking force controlunit so that the brake pad pressure is controlled in response to abraking force command value.

[0061] (2) In (1) above, assuming that the driving unit of fluidpressure type is used for the braking apparatus, the pad clearancesetting unit specifies the volume of the brake fluid corresponding tothe pad clearance returned to the brake fluid source from the paddriving unit with reference to the state where the brake pads are incontact with the brake rotor thereby to maintain a constant padclearance, while the pad position control unit supplies the brake fluidof the volume corresponding to the pad clearance to the pad drivingmeans.

[0062] (3) In (2) above, the pad clearance setting unit and the padposition control unit use as a command value the revolution speed of themotor for driving the pump in order to specify the volume correspondingto the pad clearance.

[0063] (4) In (2) above, the pump driving torque generated in responseto a command from the pad position control unit is larger than the pumpdriving torque generated in response to a command from the braking forcecontrol unit based on the braking force command value.

[0064] (5) In (1) or (2) above, the pad driving unit includes a padposition sensor, and the pad clearance setting unit turns back the padposition by an amount corresponding to a specified clearance based onthe output of the pad position sensor with reference to an originconstituted of the position where the brake pads are in contact with thebrake rotor thereby to maintain a predetermined pad clearance. The padposition control unit, on the other hand, restores the pad position tothe origin based on the output of the pad position sensor.

[0065] In (1) above, a predetermined value of the pad clearance is setfor initialization of the braking apparatus and termination of brakingoperation, while the brake pads are advanced by a distance correspondingto the pad clearance into contact with the brake rotor at the time ofstarting the braking operation. This advance of the brake pads is equalto the backward movement of the brake pads for setting the pad clearanceat the time of initialization. As a result, the position at which thebrake pads come into contact with the brake rotor can be determinedwithout any sensor for measuring the brake pad position. Once thecontacting position is determined, the motor driving power can bereduced immediately before the contact to softly seating the brake padseven when the brake pads are driven at maximum speed in order to shortenthe free-run distance. Thus, it is possible to avoid the sharp increasein the contacting force of the brake pads, i.e. the sharp increase inthe braking force due to the inertia or the delayed control of theactuator.

[0066] In (2) above, on the other hand, as in (1), the pump drivingforce is reduced immediately before the brake pads come into contactwith the brake rotor thereby to softly seat the brake pads. In thiscase, too, the sharp rise in the brake fluid pressure, i.e. the sharprise in the braking force can be avoided.

[0067] In (3) above, the pump revolution speed corresponding to theclearance in the fluid pressure type 20 of the braking apparatus is usedas a command value, and the pump revolution speed is compared with thecommand value, thereby making it possible to stabilize the brake fluidpressure with a simple structure.

[0068] In (4) above, the pump driving torque for 25 bringing the brakepads into contact with the brake rotor at the time of starting thebraking operation is set higher than the pump driving torque at the timeof pressure control, thereby making it possible to shorten the vehiclefree-run distance before the brake begins to work.

[0069] In (5) above, the provision of a pad position sensor leads to theeffect of managing the position where the brake pads come into contactwith the brake rotor for high responsiveness and stabilization of thebraking force.

[0070] As described above, the high responsiveness and stabilization ofthe braking force prevents the spin and drift of the vehicle andstabilizes the vehicle behavior and motion. Also, in the automaticbraking operation using an advanced cruise control unit for measuringand maintaining a constant distance with the vehicle running ahead or anobstacle lying ahead, the braking force can be generated smoothly andquickly. Also, in the advanced cruise control unit for measuring thespacing or headway to a preceding vehicle or an obstacle, a padclearance is closed in advance in the case where the braking operationby the driver can be predicted, thus making it possible to respondquickly to the driver's demand for braking.

[0071] It will thus be understood from the foregoing description thataccording to this invention, the delay time of braking force generationcan be shortened and the accuracy of the braking control can be improvedby controlling the braking position and the braking force.

What is claimed is:
 1. A braking apparatus comprising: a first contact unit disposed on a rotary shaft of a wheel; a second contact unit for contacting the first contact unit to generate a friction force; an actuator for moving and pressing the second contact unit toward the first contact unit; a control unit for controlling the operation of the actuator; a first memory, included in the control unit, which stores a first distance value set as a distance between the first and second contact units, the control unit controlling the actuator to cause the distance between the first and second contact units to be a second distance value which is narrower than the first distance value at a time of braking operation; a second memory, included in the control unit, which stores a third distance value different from the first and second distance values; and a switching unit which selects either one of the first and third distance values stored in the first and second memories, the distance between the first and second contact units being set to the third distance value by the control unit when the third distance value is selected.
 2. A braking apparatus according to claim 1, wherein the second distance value is a value of the distance between the first and second contact units when the first and second contact units are almost in contact with each other.
 3. A braking apparatus according to claim 1, wherein the third distance value is larger than the first distance value.
 4. A braking apparatus according to claim 2, wherein the third distance value is larger than said first distance value.
 5. A braking apparatus according to claim 1, wherein the first and second memories are included in a single memory device.
 6. A braking apparatus according to claim 1, wherein the first and second memories are separate memory devices.
 7. A braking apparatus comprising: a first contact unit disposed on a rotary shaft of a wheel; a second contact unit for contacting the first contact unit to generate a friction force; an actuator for moving and pressing the second contact unit toward the first contact unit; a control unit for controlling the operation of the actuator; a first memory, included in the control unit, which stores a first distance value set as a distance between the first and second contact units; the control unit executing a position control for controlling the actuator to cause the distance between the first and second contact units to be a second distance value which is narrower than the first distance value at a time of braking operation; the control unit executing a braking force control for controlling a pressure force of the second contact unit against the first contact unit by the actuator according to a required pressure; a second memory, included in the control unit, which stores a third distance value different from the first and second distance values; and a switching unit which selects either one of the first and third distance values stored in the first and second memories, the distance between the first and second contact units being set to the third distance value by the control unit when the third distance value is selected.
 8. A braking apparatus according to claim 7, wherein the second distance value is a value of the distance between the first and second contact units when the first and second contact units are almost in contact with each other.
 9. A braking apparatus according to claim 7, wherein the third distance value is larger than the first distance value.
 10. A braking apparatus according to claim 7, wherein the first and second memories are included in a single memory device, or separate memory devices.
 11. A braking apparatus comprising: a brake disk arranged on a rotary shaft of a wheel; at least a brake pad; a brake cylinder including a piston arranged to press the brake pad against the brake disk under the pressure of a brake fluid; a pump arranged to supply the brake fluid to the brake cylinder; and a controller arranged to regulate input of the brake fluid to the brake cylinder for maintaining a preset clearance between the brake pad and the brake disk, and for performing a position control operation to move the brake pad towards the brake disk and a force control operation to press the brake pad against the brake disk in accordance with a required braking force, the controller comprising: a pad clearance setting unit arranged to set a preset clearance between the brake pad and the brake disk; a pad position control unit arranged to move the brake pad to a position substantially in contact with the brake disk; a brake force control unit arranged to control the pressure to press the brake pad against the brake disk; and a switching unit arranged to switch between the brake force control unit, the pad position control unit and the pad clearance setting unit, to regulate the input of the brake fluid to the brake cylinder.
 12. A braking apparatus according to claim 11, further comprising: a pressure sensor arranged to sense the pressure of the brake fluid applied to the brake cylinder and generate an output to the controller; and a flow rate detector arranged to detect the flow rate of the brake fluid applied to the brake cylinder and generate an output to the controller.
 13. A braking apparatus according to claim 11, wherein a driving torque of the pump generated in response to a pad position command is larger than the driving torque of the pump generated in response to a braking force command.
 14. A braking apparatus according to claim 11, wherein the controller contains a computer program to perform the functionality of the pad clearance setting unit, the pad position control unit, the braking force control unit, and the switching unit.
 15. A braking apparatus comprising: a brake disk arranged on a rotary shaft of a wheel; at least a brake pad; a mechanism arranged to control movement of the brake pad relative to the brake disk, including to press the brake pad against the brake disk; and a controller arranged to control the operation of the mechanism, including to maintain a preset clearance between the brake pad and the brake disk, and to perform a position control operation to move the brake pad towards the brake disk and a force control operation to press the brake pad against the brake disk in accordance with a required braking force, the controller comprising: a pad clearance setting unit arranged to set a preset clearance between the brake pad and the brake disk; a pad position control unit arranged to move the brake pad to a position substantially in contact with the brake disk; a brake force control unit arranged to control the pressure to press the brake pad against the brake disk; and a switching unit arranged to switch between the brake force control unit, the pad position control unit and the pad clearance setting unit, to control movement of the brake pad relative to the brake disk.
 16. A braking apparatus according to claim 15, wherein the mechanism controlling the movement of the brake pad relative to the brake disk comprises: a brake cylinder including a piston arranged to press the brake pad against the brake disk under the pressure of a brake fluid; a pump arranged to supply the brake fluid to the brake cylinder; a pressure sensor arranged to sense the pressure of the brake fluid applied to the brake cylinder and generate an output to the controller to regulate the pressure of the brake fluid; and a flow rate detector arranged to detect the flow rate of the brake fluid applied to the brake cylinder and generate an output to the controller to regulate the pressure of the brake fluid.
 17. A braking apparatus according to claim 16, wherein a driving torque of the pump generated in response to a pad position command is larger than the driving torque of the pump generated in response to a braking force command.
 18. A braking apparatus according to claim 17, wherein the controller contains a computer program to perform the functionality of the pad clearance setting unit, the pad position control unit, the braking force control unit, and the switching unit.
 19. A braking apparatus according to claim 15, wherein the mechanism controlling the movement of the brake pad relative to the brake disk comprises: an actuator arranged to press the brake pad against the brake disk; and a detector arranged to detect the position of the brake pad relative to the brake disk and generate an output to the controller to control the operation of the actuator.
 20. A braking apparatus according to claim 19, wherein the controller contains a computer program to perform the functionality of the pad clearance setting unit, the pad position control unit, the braking force control unit, and the switching unit. 